Fri., Jan 27, 2006

Don't forget the Assignment #1 1S1P reports are due on Monday.

There is an optional assignment due next Wednesday.  The Practice Quiz is also next Wednesday.  The reviews for the Practice Quiz will all be held in Chavez 301.  Dates and times of the reviews are on the Practice Quiz Study.

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This is page 26 in the photocopied notes redrawn on a blank sheet of paper.

As you move upward from the ground pressure decreases by 100 mb in both figures.  Both layers contain the same amount of air (10% of the air in the atmosphere).  That air is found in a smaller volume in the figure at left (the layer is thinner).  This means the air at left is denser than the air at right.  The drop in air pressure in the layer at left occurs in a shorter vertical distance than in the air layer at right.  That is a more rapid rate of pressure decrease with distance than in the layer at right.

The rate of pressure decrease with altitude is higher in the dense air at left than in the lower density air at right.

This is a fairly subtle but important concept.  We will use it when we try to understand the intensification of hurricanes later in the semester.


Air pressure is a force that pushes downward, upward, and sideways.  The bottom person in the people pyramid below must push upward with enough force to support the other people.  The air pressure in the four tires on your automobile push down on the road (you would feel that if the car ran over your foot) and push upward with enough force to keep the 1000 or 2000 pound vehicle off the road.




I am going to redraw the two figures from p. 35 in the photocopied notes that described the class demonstration of upward air pressure force.  Hopefully this will make for a clearer explanation of the demonstration.


At left, gravity, the red arrow, pulls downward on the water balloon.  The 10 is the strength of the gravitational force (I just made this number up, don't worry about the units either).  Air pressure pushes upward on the bottom of the balloon with strength 15,  slighly weaker forces (14.5) push sideways, and at the top of the balloon air pressure pushes downward with strength 14.  Pressure decreases with increasing altitude, that is why the pressure forces get weaker as you move from the bottom to the top of the balloon. 

Notice the two sideways forces cancel each other out.  The upward pointing pressure force is slightly stronger than the downward pressure force.  They don't quite cancel each other out, you are left (as shown in the figure at right) with an upward pointing pressure difference force of strength 1.  Gravity (10) is stronger than the pressure difference force (1) and the water balloon falls.

Now we'll look at the situation for the water in an upside down wine glass


The figure at left shows the wine glass (the stem of the glass has been left off the drawing), the plastic cover, and the pressure and gravity forces. 

You can split this into two parts:  (1) gravity pulling downward and the pressure force pushing upward on the cover and water in the glass and (2) the pressure force pushing downward and sideways on the glass (which the instructor was holding in his hand).   Now notice that the 15 unit pressure force pushing up on the water is greater than the 10 units of gravity pulling downward.  The upward pressure force now can keep the water from falling out of the glass.

Here's another situation where pressure and gravity forces are battling each other for dominance, a balloon.



This is like a water balloon except that the balloon is now filled with a gas like air or helium.  This gas is much lighter than water.  The gravity force will be much weaker and the upward pressure difference force (the difference between the upward point force at the bottom of the balloon and the downward pointing force at the top of the balloon) will sometimes end up stronger than gravity.


Three layers of air in the atmosphere are shown above (each layer contains the same amount of air, 10% of the air in the atmosphere).  The layer at the ground and at the bottom of the atmosphere is "squished" by the weight of the air above.  Squeezing all of this air into a thin layer or small volume increases the air's density.

 The next layer up is also squished but not as much as the bottom layer.  The density of the air in the second layer is lower than in the bottom layer.  The air in the 3rd layer has even lower density.  It is fairly easy to understand that air density decreases with increasing altitude. 

This figure also reminds you that air pressure decreases with increasing altitude.

If you look carefully you see again that the most rapid rate of pressure decrease with increasing altitude occurs in the thin layer of high density air next to the ground.